This replaces abandoned PR #14824 by @ekbiker, and gives it some love.

Changes since last roll:
```
ed303c628 Roll dart sdk again. Previous roll required 23ae4fa098 revert. (#4966)
8cd272733 Revert "Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)" (#4965)
9199b40f2 Revert "Support multiple shells in a single process. (#4932)" (#4964)
6baff4c82 Support multiple shells in a single process. (#4932)
31c5bb427 Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)
c8e4c6984 Avoid copying the contents of large platform message responses (#4947)
5ff527295 Update to use new vulkan GrBackendRenderTarget ctor. (#4962)
0c8993a1a Update to use new vulkan GrBackendRenderTarget ctor (part 2) (#4963)
132ebdda8 Revert "Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)" (#4961)
11882ab9e Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)
```

Add consts

* Add disable Cupertino Switch example

* keep trailing commas consistent

* fix Cupertino Switch example linter error

This is not a grand refactor yet, it's just cleaning up what we have
already, so that people who keep using this API (e.g. dialogs) have
something coherent to deal with.

The major changes are that Navigator and NavigatorState have the same
API now, that most of the examples use `<void>` instead of `<Null>`,
that the navigator observer can see replaces, and that the `settings`
is moved from ModalRoute to Route. I also cleaned up some of the API
documentation.

* Gallery scaffolding

* Started RenderSliver

* demo and initial hookup

* Cleaned up demo more and scaffolding basic sliver->widget communication structure.

* works

* states and default indicator building works

* start adding docs

* added an alignment setting optimized the sliver relayout mechanism

* tested a default bottom aligned sized indicator

* Added a bunch of tests

* more fixes and more tests

* Finished the tests

* Add docs

* Add more doc diffing wrt material pull to refresh

* Mention nav bar synergy

* add more asserts

* review 1

* Fix mockito 2 / dart 2 / strong typed tests

* review

* Remove the vscode config

* review

* replace child parameter with builder on showDialog

* change builder parameter to WidgetBuilder

* mark child as deprecated and expand documentation to cover this and how builder should be used

* tidy comments and address some feedback

* phrasing

* move space to prev line and add //ignore comments for deprecated member use

* address comments and fix it's its

* update code samples

* adds semicolon to code snippets

* spelling: accommodate

* spelling: allotted

* spelling: anonymous

* spelling: artificial

* spelling: associated

* spelling: asset

* spelling: button

* spelling: canvas

* spelling: compatibility

* spelling: coverage

* spelling: condition

* spelling: decoration

* spelling: deferring

* spelling: diameter

* spelling: direction

* spelling: displacement

* spelling: dropdown

* spelling: needing

* spelling: environment

* spelling: exited

* spelling: expansion

* spelling: explore

* spelling: families

* spelling: horizontal

* spelling: increment

* spelling: indices

* spelling: internationalization

* spelling: labrador

* spelling: localizations

* spelling: midflight

* spelling: milliseconds

* spelling: minimum

* spelling: multiple

* spelling: multiplication

* spelling: navigator

* spelling: overridden

* spelling: package

* spelling: performance

* spelling: platform

* spelling: porsche

* spelling: position

* spelling: preceded

* spelling: precede

* spelling: precedence

* spelling: print

* spelling: property

* spelling: readily

* spelling: reproducibility

* spelling: rounded

* spelling: scroll

* spelling: separate

* spelling: separator

* spelling: services

* spelling: specific

* spelling: specify

* spelling: synchronously

* spelling: through

* spelling: timeout

* spelling: triangle

* spelling: trivial

* spelling: unusual

* spelling: then

* spelling: vertically

* spelling: visible

* spelling: visited

* spelling: voice

* Add selection haptic feedback to Cupertino Pickers

* review

* don't haptic on android

Discovered during an a11y review of the gallery.

* controller, position and test

* Make controllers swappable

* WIP

* Create a ListWheelScrollPhysics

* Created picker and gallery demo and testing now

* Works. Ready to document and test.

* Document and add tests. Make the scroll controller more generic.

* minor cleanup

* review

* review

* fix tests

* stop using TransformLayers for now

Adds safe areas around:
1. The body of the colour swatch view (tab 1)
2. The chat header (tab 2)
3. The chat bubbles (tab 2)
4. The 'Sign In' launcher button (tab 3)

Let translucent Cupertino bars have its scaffold children automatically pad their heights - second try (#13440)

* Let translucent Cupertino bars have its scaffold children automatically pad their heights (#13194)

* Let lists automatically add sliver padding from media query. Translucent nav and tab bars leave behind media query paddings in scaffolds.

* tests

* const lint

* Rename base abstract class to generalized ObstructingPreferredSizeWidget

* review

* More docs and comments from #13317

Revert "Let translucent Cupertino bars have its scaffold children automatically pad their heights (#13194)" (#13317)

This reverts commit 5c4ffa13.

Extracts Tab2ConversationRow class that consolidates widget tree for a
chat message in the Cupertino navagation chat demo. This refactoring
simplifies adding SafeArea support to handle iOS 11 safe areas in a
followup patch.

* Let lists automatically add sliver padding from media query. Translucent nav and tab bars leave behind media query paddings in scaffolds.

* tests

* const lint

* Rename base abstract class to generalized ObstructingPreferredSizeWidget

Fixes the remaining known issues with widgets supporting RTL.

* Built first tab

* Small additions

* started tab 3

* Need color arithmetics

* tab 2 built

* finalize

* lint and tests

* review

* Reapply docs after rebase

* use color.computeLuminance

* linter

* nit

Alignment will eventually replace FractionalOffset.

* is default

* hit target

* correct test name

* review note

Fixes #9757

* more widget const constructors

* prefer const constructors

* address review comments

Also switches to single quotes for consistency with surrounding code.

Aggressively apply the const lint.

The latter is the proper spelling.

Fixes #8883

* prefer const constructors

* fix double throw

* Add cupertino to gallery and add CupertinoButto

* Use single quotes

* Add disabled state

* Some review notes

* Make button animation more responsive and tweak timing

* Renamed things Cupertino

* Button with background, move cupertino demos, move material demos

* Move 2 level list too

* Refactor various demo route names

* Some review notes

* More reviews and add test

* Linter as

* Move private constant up